Zoe Chace and Robert Smith are reporting from European borders this week. This is the first story in a four-part series.

A metal strip on the floor of Eurode Business Center marks the border between Germany and the Netherlands.

On one side of the building, there's a German mailbox and a German policeman. On the other side, a Dutch mailbox and a Dutch policeman.

The building was supposed to make it easy to work in both countries. But it's also a reminder of how the European dream isn't yet a reality.

The border is open enough that a single building can span it. No border guards, no checkpoints. But a letter sent from the German side of the building takes a week to get to the Dutch side.

A computer security company called Alunsa has offices on both sides of the building. On one side, Alunsa employees call German customers on German phones. On the other side, it's all Dutch. Raimond Potgens, the company's CEO, has two offices, one on either side of the border. He carries his laptop back and forth all day long.

The reason for all the back and forth: Taxes. "As long as Holland and Germany want all their money, it's not one union," Potgens says.

Jan Schlievert, an EU lawyer who also works in the Eurode, agrees with Potgens. "Money is always the turning point for all these laws," he says.

Schlievert tells the story of a Belgian carpenter who spent his days doing carpentry in Germany, then drove home to spend the night in Belgium. The German company he worked for paid for the car.

Belgium wanted to tax the car because it spent the night in a Belgian driveway. Germany wanted to tax the car because it spent the day on German roads. The fight was such a big deal to both countries that they went to court to figure out who got to tax the car. (Germany won.)

This is how a united Europe is being created. Slowly, case by case, one Belgian carpenter at a time.

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STEVE INSKEEP, HOST:

Let's cross the English Channel now, and enter the eurozone. Many European nations adopted a single currency years ago. And during the debt crisis, they've been struggling desperately - at great cost - to keep the euro from unraveling.

RENEE MONTAGNE, HOST:

The euro was supposed to help unify the continent. It's part of a much larger, generations-long project of lowering barriers at European borders.

INSKEEP: But after all this time, the border lines have proven surprisingly durable. Consider the border where we find NPR's Planet Money team.

ZOE CHACE, BYLINE: I'm Zoe Chace. I'm in an office building in the Netherlands.

ROBERT SMITH, BYLINE: And I'm Robert Smith, and I'm in an office building in Germany - and it's actually the same building. There's a borderline that runs right down the middle of the - what do we call this?

HANS HOEVER: We call it Eurode Business Center.

CHACE: Hi. See? We're right next to each other.

SMITH: We're right here.

CHACE: The only sign that we're right on the border, is this metal line embedded in the floor of the building. It's the historical divide between Holland and Germany. Our guide, Hans Hoever, points it out.

SMITH: Where's the border guard?

HOEVER: Border guard? We haven't any - (LAUGHTER) longer.

CHACE: The Eurode Business Center looks like your typical modern office building you might see off the freeway. Just a few floors; dozens of different companies.

SMITH: Everything in this building is twinsies. Here on the German side, there's a German mailbox, German policemen, rows of German companies.

CHACE: Over here: a Dutch mailbox, Dutch policemen, Dutch companies - a few feet away from each other.

SMITH: This building is set up to make it easy to work in multiple countries. But the center's very existence shows how far Europe has to go. Business can be very different, depending on which hallway you go down. For instance, I'm in the German wing...

(SOUNDBITE OF KNOCKING)

SMITH: ...where I find Raimond Potgens, CEO of Alunsa. They sell risk-management software to big companies, like Adidas. And at first, it seems like a normal German company, taking German phone calls on German phones.

(SOUNDBITE OF PHONE RINGING)

RAIMOND POTGENS: (Speaking German)

SMITH: But then, Potgens starts talking about his other office - his Dutch office on the other side of the building, just down the hall, where Dutch customers call on his Dutch phone.

POTGENS: (Speaking Dutch)

CHACE: It's not that weird for companies to have German branches and Dutch branches, but they aren't usually 40 yards apart. And it's not usually the same workers carrying their laptops back and forth across the border, every single day.

SMITH: I thought the whole point of having the European Union was so that you didn't have to do crazy stuff like this.

POTGENS: Yeah. But as long as Holland and Germany want all their money, it's not one union.

SMITH: Ah. And there's the problem. The European Union didn't create a central government, and so each country fights for their cut of the taxes. Each country has their own, complicated tax code and when Potgens does the math, he thinks it will be cheaper for him to keep two different offices, just across the border from each other.

CHACE: This nationalist urge to grab all the revenue they can - it's not just taxes. Remember those twin mailboxes standing right next to each other, in two different countries?

SMITH: They aren't just symbolic. If you try to send a letter from one end of this building to the other end, and put it in the wrong box...

POTGENS: The letter will go from here to (unintelligible) with airplane to Frankfurt.

CHACE: It wasn't supposed to be like this. The whole idea of European integration, is the smoothing of the borders.

SMITH: And right in the center of this building, we meet the guy who's trying to eliminate the bumps from one country to another.

JAN SCHLIEVERT: Money is always the turning point for all these laws.

CHACE: Jan Schlievert is a German EU lawyer. When people get stuck between bickering European countries with different laws, they come to him.

SMITH: For instance, Jan tells us the story of the Belgian carpenter. The carpenter spends the day doing carpentry on my side of the border, here in Germany.

CHACE: And then at night, drives the company car home to this side - Belgium.

SCHLIEVERT: And then there's a problem because the Belgium taxation office might say, well, you're here in Belgium as a private citizen, driving that car.

CHACE: So Belgium wants to tax the car because it spends the night in a Belgian driveway.

SMITH: And Germany wants to tax the car because it spends the day on German roads. They had to go all the way to the European court, to figure this one out. Germany won.

CHACE: I know it might seem minor, but a unified Europe is being created like this - case by case. Jan Schlievert says they have a saying.

SCHLIEVERT: (Speaking foreign language) - a Europe from the regions.

CHACE: A Europe from the regions.

SMITH: Of course, when the United States created a country from a bunch of regions, it had a central government to help.

CHACE: And a couple hundred years to do it.

SMITH: Europe has buildings like this one, perched between two nations and taking it one car, one mailbox, one problem at a time.